Slow Etch Rate Research Articles

Fabrication of ultrahigh aspect ratio Si nanopillar and nanocone arrays

High aspect ratio (HAR) structures have many promising applications such as biomedical detection, optical spectroscopy, and material characterization. Bottom-up self-assembly is a low-cost method to fabricate HAR structures, but it remains challenging to control the structure dimension, shape, density, and location. In this paper, an optimized top-down method using a combination of pseudo-Bosch etching and wet isotropic thinning/sharpening is presented to fabricate HAR silicon (Si) nanopillar and nanocone arrays. To achieve these structure profiles, electron beam lithography and reactive ion etching were carried out to fabricate silicon pillars having a nearly vertical sidewall, followed by thinning or sharpening by wet etching with a mixture of hydrofluoric (HF) acid and nitric acid (HNO3). For the dry etching step using the pseudo-Bosch process, the sidewall angle is largely dependent on the SF6/C4F8 gas flow ratio, and it was found that a vertical profile can be attained with a ratio of 22/38. For the wet etching process, a very large HNO3/HF volume ratio is shown to give smooth etching with a slow and controllable etching rate. The final structure profile also depends on the pattern density/array periodicity. When the array period is large, silicon nanopillar is thinned down, and its aspect ratio can reach 1:135 with a sub-100 nm apex. When the pillar array becomes very dense (periodicity much smaller than height), a very sharp nanocone structure is obtained after wet etching with an apex diameter under 20 nm.

Diamond membrane production: The critical role of radicals in the non-contact electrochemical etching of sp2 carbon

Sub-micrometre single crystal diamond membranes are of huge importance for next generation optical, quantum and electronic device applications. Electrochemical etching has proven a critical step in the production of such membranes. Etching is used to selectively remove a very thin layer of sub-surface sp2 carbon, prepared by ion implantation in bulk diamond, releasing the diamond membrane. Due to the nanosized dimensions, etching is typically carried out using non-contact electrochemistry in low conductivity solutions (bipolar arrangement) which whilst effective, results in extremely slow etch rates. In this work, a new method of non-contact electrochemical etching is presented which uses high conductivity, high concentration, fully dissociated aqueous electrolytes. Careful choice of the electrolyte anion results in significant improvements in the sp2 carbon etch rate. In particular, we show both chloride and sulfate electrolytes increase etch rates significantly (up to × 40 for sulfate) compared to our measurements using the current state-of-the-art solutions and methodologies. Electron paramagnetic resonance experiments, recorded after the electrode potential has been switched off, reveal sizeable hydroxyl radical concentrations at timescales > 107 longer than their lifetime (≤μs). These measurements highlight the importance of electrochemically initiated, solution chemistry radical generation and regeneration pathways in high concentration sulfate and chloride solutions for nano-etching applications.

High-Quality Etching of GaN Materials with Extremely Slow Rate and Low Damage

High-quality gallium nitride etching is highly desirable in electronic device fabrications. For the GaN base devices, the electronic properties largely depended on the etching induced surface damages. To overcome this, a controllable GaN etching method was developed using inductively coupled plasma reactive ion etching (ICP-RIE) by controlling radio frequency (RF) power, and DC bias. The etching rate, DC bias, and root-mean-square surface roughness were measured as a function of bias power under different RF, 40 and 13.56 MHz. The effects of ICP power and chlorine to argon percentage were systematically studied. An extremely slow etching rate and low-damage surface were achieved by reducing DC bias power to 25 W under RF 40 MHz. Ni|Au Schottky diodes were fabricated and characterized. The diode fabricated on the 40-MHz RF etching GaN surface has a much lower ideality factor and higher barrier height than non-etched GaN and RF 13.56 MHz etching GaN.

A Surface Micromachining: HNA Etchant for Stiction-free Release of Micro/Nanomechanical Structures

We report a simple and efficient silicon wet etchant HNA (Hydrofluoric acid, Nitric acid, Acetic acid) for the stiction free release of silicon nitride/metal micro/nanomechanical structures. The HNA concentration was varied with the aim of developing a slow etch rate, which could be utilized to suspend the micron and sub-micron cantilever and fixed beam structures. The etch rate was found to decrease with decrease in HF and increase in HNO 3 concentrations. Smooth surface and high selectivity are obtained in case of high HNO 3 content. The obtained etchant with a slow etch rate of 440 nm/minute was successfully utilized for the release of silicon nitride cantilevers and fixed beam structures of 500 nm, 2 µm and 5 µm widths with varying lengths. The resonance frequency of suspended cantilevers characterized by Laser Doppler Vibrometer is in close agreement with the COMSOL simulated resonating frequencies proving that the etching process has been precise without changes to cantilever dimensions. In addition, the generated etchant was successfully used in releasing chrome/gold nanobeams as well without any damage to the metal layers.

Development of SiC Etching by Chlorine Fluoride Gas

Amorphous SiC and 4H-SiC were etched by chlorine fluoride (ClF), chlorine trifluoride (ClF3), fluorine (F2) and chlorine (Cl2) gases, in order to evaluate the etching capability of various reactive gases. The gaseous byproducts of SiC etching were observed by the quadrupole mass spectrometry. The ClF showed slow etching rate of amorphous SiC and 4H-SiC, while ClF3 and F2 quickly etched them. By the ClF gas, the etching rate of amorphous SiC was 120 nm/min at 400 °C. The ClF gas was expected to be suitable for a shallow etching, because of its moderate etching rate even at high temperatures.

Comparison of chemical stability and corrosion resistance of group IV metal oxide films formed by thermal and plasma-enhanced atomic layer deposition

The wide applications of ultrathin group IV metal oxide films (TiO2, ZrO2 and HfO2) probably expose materials to potentially reactive etchants and solvents, appealing for extraordinary chemical stability and corrosion resistance property. In this paper, TiO2 ultrathin films were deposited on Si at 200 °C while ZrO2 and HfO2 were grown at 250 °C to fit their growth temperature window, by thermal atomic layer deposition (TALD) and plasma-enhanced ALD (PEALD). A variety of chemical liquid media including 1 mol/L H2SO4, 1 mol/L HCl, 1 mol/L KOH, 1 mol/L KCl, and 18 MΩ deionized water were used to test and compare chemical stability of all these as-deposited group IV metal oxides thin films, as well as post-annealed samples at various temperatures. Among these metal oxides, TALD/PEALD HfO2 ultrathin films exhibit the best chemical stability and anti-corrosion property without any change in thickness after long time immersion into acidic, alkaline and neutral solutions. As-deposited TALD ZrO2 ultrathin films have slow etch rate of 1.06 nm/day in 1 mol/L HCl, however other PEALD ZrO2 ultrathin films and annealed TALD ones show better anti-acid stability, indicating the role of introduction of plasma O2 in PEALD and post-thermal treatment. As-deposited TiO2 ultrathin films by TALD and PEALD are found to be etched slowly in acidic solutions, but the PEALD can decrease the etching rate of TiO2 by ~41%. After post-annealing, TiO2 ultrathin films have satisfactory corrosion resistance, which is ascribed to the crystallization transition from amorphous to anatase phase and the formation of 5% Si-doped TiO2 ultrathin layers on sample surfaces, i.e. Ti-silicate. ZrO2, and TiO2 ultrathin films show excellent corrosion endurance property in basic and neutral solutions. Simultaneously, 304 stainless steel coated with PEALD-HfO2 is found to have a lower corrosion rate than that with TALD-HfO2 by means of electrochemical measurement. The pre-treatment of plasma H2 to 304 stainless steel can effectively reduce interfacial impurities and porosity of overlayers with significantly enhanced corrosion endurance. Above all, the chemical stability and anti-corrosion properties of IV group metal oxide coatings can be improved by using PEALD technique, post-annealing process and plasma H2 pre-treatment to substrates.

Fracture strength and microstructural study of ultrathin Si die with Cu backside layer diced with picosecond laser

Mechanical and plasma dicing of Si wafers with a backside metal layer poses serious challenges in quality and manufacturing cost. Mechanical dicing through the metal layer causes blade clogging and damage, which results in severe Si die chipping and cracks. The cost of plasma dicing is high as additional photolithography steps are required for etching the metal layer, in addition to the extremely slow metal etch rate. Laser dicing is promising and is currently used to singulate thin Si wafers. In this paper, picosecond laser dicing of 20 μm Si dies with 0–30 μm backside Cu was found to be feasible. Utilizing an improved three-point bending test method, the frontside and backside characteristic fracture strengths of ultrathin Si dies with varying backside Cu thickness were measured. The die types with backside Cu show an average of 17% higher frontside characteristic fracture strength, and an average of 13% higher backside characteristic fracture strength, compared to the die type without backside Cu. The die sidewall microstructures, defects, and compositions have been characterized in detail by transmission electron microscopy, and their effect on mechanical strength is discussed.

Deterministic integration and optical characterization of telecom O-band quantum dots embedded into wet-chemically etched Gaussian-shaped microlenses

In the present study, we report on the deterministic integration of quantum dots, emitting in the telecom O-band, into wet-chemically fabricated Gaussian-shaped microlenses which exhibit a surface quality comparable to epi-ready wafers. The slow wet-chemical etching rate enables us to gain control of the lens aspect-ratio and the vertical position with respect to the quantum dot, allowing us to engineer the far field shape to better match the acceptance profile of single-mode fibers. Maximum light enhancement values of around 10 to 16 could be achieved for collection numerical apertures from 0.6 to 0.2, respectively. The current results constitute an important step forward in transferring state-of-the-art performances achieved in the near-infrared regime to the key wavelengths for long distance fiber communication.

Highly sensitive 4H-SiC pressure sensor at cryogenic and elevated temperatures

The slow etching rate of conventional micro-machining processes is hindering the use of bulk silicon carbide materials in pressure sensing. This paper presents a 4H-SiC piezoresistive pressure sensor utilising a laser scribing approach for fast prototyping a bulk SiC pressure sensor. The sensor is able to operate at a temperature range from cryogenic to elevated temperatures with an excellent linearity and repeatability with a pressure of up to 270 kPa. The good optical transparency of SiC material allows the direct alignment between the pre-fabricated piezoresistors and the scribing process to form a diaphragm from the back side. The sensitivities of the sensor were obtained as 10.83 mV/V/bar at 198 K and 6.72 mV/V/bar at 473 K, which are at least a two-fold increment in comparison with other SiC pressure sensors. The high sensitivity and good reliability at either cryogenic and elevated temperatures are attributed to the profound piezoresistive effect in p-type 4H-SiC and the robust p-n junction which prevents the current from leaking to the substrate. This indicates the potential of utilising the laser scribing approach to fabricate highly sensitive bulk SiC pressure sensors for harsh environment applications.

A differential Hall effect measurement method with sub-nanometre resolution for active dopant concentration profiling in ultrathin doped Si1-x Ge x and Si layers.

In this paper, we present an enhanced differential Hall effect measurement method (DHE) for ultrathin Si and SiGe layers for the investigation of dopant activation in the surface region with sub-nanometre resolution. In the case of SiGe, which constitutes the most challenging process, we show the reliability of the SC1 chemical solution (NH4OH/H2O2/H2O) with its slow etch rate, stoichiometry conservation and low roughness generation. The reliability of a complete DHE procedure, with an etching step as small as 0.5 nm, is demonstrated on a dedicated 20 nm thick SiGe test structure fabricated by CVD and uniformly doped in situ during growth. The developed method is finally applied to the investigation of dopant activation achieved by advanced annealing methods (including millisecond and nanosecond laser annealing) in two material systems: 6 nm thick SiGeOI and 11 nm thick SOI. In both cases, DHE is shown to be a uniquely sensitive characterisation technique for a detailed investigation of dopant activation in ultrashallow layers, providing sub-nanometre resolution for both dopant concentration and carrier mobility depth profiles.

Formation of 45° Silicon (110) Surface Using Triton X-n Surfactants in Potassium Hydroxide for Infrared Applications

Silicon (Si) micromirrors are an integral feature for many micro-optomechanical systems (MOEMS). Such mirrors are generally wet etched in alkaline solution at elevated temperature. For 90° beam steering applications, 45° slanted Si (110) plane is the prime choice fabricated with the incorporation of tensioactive surfactants. Here, Triton-Si and Triton-hydroxide (OH−)/H2O interaction using varying hydrophilic chain length Triton (X-45, X-100 and X-405) were investigated. The surfactant concentration was varied from 0 to 1000 ppm in potassium hydroxide (KOH). Triton molecules were shown to adsorb preferentially on (110) than on (100) surface. Longer chain length Triton hampered OH− access to Si surface resulting in slower etch rate. In contrast, contact angle measurement suggested that shorter Triton interfaced better with Si surface. Later, Si wafers etched in Triton 10 ppm – KOH were examined. The measured output for (110)X-45, (110)X-100, (110)X-405 and polished Si wafer reference (Rq < 5Å) mirrors were 0.58, 0.76, 0.72 and 1.25 mW, respectively. Subsequently, Si-SiO2 thin film in [HLHL]2-substrate configuration was fabricated. Broadband micromirror for use in 3.0–5.5 μm spectrum range was experimentally realized with reflected efficiency of 73%.

Measuring Carrier Density in InP‐based Materials by Using EDTA Solution in Electrochemical Capacitor Voltage Testing

The quick test and evaluation of carrier density and its distribution is an important part in the fabrication of photonic integrated devices. Ammonium tartrate is commonly used as the corrosion fluid when electricity chemistry capacitance voltage analysis (ECV) was used to measure the carrier density of InP‐based epitaxial wafer. However, it has the shortcomings of trivial preparation process, volatilization, slow etching rate, which seriously influence the efficiency of the test. Hence, we have adopted a new solution, which is called as ethylenediamine tetraacetic acid disodium (EDTA). We experimentally demonstrated that the EDTA solution has the advantages of convenience, high efficiency and good stability. And, it can significantly reduce the etching time, thus improving the ECV test efficiency and reducing the production cycle and device fabrication cost.

Wafer Thinning for Advanced Packaging Applications

Driven largely by the growing need for more data, increased functionality, and faster speeds, consumer electronic devices have sparked a revolution in IC design. As it becomes increasingly more expensive and technically challenging to scale down semiconductor devices, Moore's law is yielding to the concept of “More than Moore”, which is driving integrated functionality in smaller and thinner packages. Packaging for 2.5D and 3D has become critical to new products requiring higher performance and increased functionality in a smaller package. The use of a Through Silicon Via (TSV) has been discussed as a method for stacking die to achieve a vertical interconnect. The high costs associated with this technology have limited TSV use to a few applications such as high-bandwidth memory and logic, slowing its adoption within the industry. Lower-cost advanced packaging concepts have been developed and are now in high-volume production. Recently, alternative methods for exploiting the z-direction have turned to variations of Fan-Out Wafer Level Packaging (FOWLP), which do not include TSVs. In many of these concepts there is a need to thin the wafer to remove all of the silicon while being selective and not etching a variety of other films that include oxides, nitrides, and metals. In addition, there can be temporary bonding adhesives and mold compounds encapsulating the chips; these must remain undamaged. Another critical element of a successful process is the ability to control the profile of the silicon etch to provide uniform removal. The single wafer wet etching techniques and advanced process control developed for TSV Reveal are applicable to these structures and provide a low-cost alternative to CMP and Plasma processes. To successfully execute the process, several characteristics must be met: the silicon overburden depth and profile need to be determined, the overburden thinning etch needs a fast sculpting etchant, and the finishing etchant needs to be selective to materials that will be exposed at the completion of the etch. In addition, the tool used to perform this sequence needs to have the correct metrology capability, along with properly chosen etchants. Similarly, it is not sufficient to know the required etch profile, the software must be able to execute a unique etch profile for each wafer. In this fashion, the finishing etch time can be kept to a minimum. This is important, as many of the selective etchants have a slow etch rate, and adhesives used do not always hold up to exposure to the chemistries involved for long periods. This paper discusses the use of wet etch wafer thinning processes for new FOWLP applications.

Plasma Etching Chemistry for Smoothening Ofultrananocrystalline Diamond Films

Ultrananocrystalline diamond (UNCD) is characterized by the dimensions of its ultra-nanoscale grains and is typically synthesized under high nucleation conditions using an argon-rich hydrogen/methane/argon gas mixture. UNCD films consist of crystalline grains of at least 95% sp3-bonded carbon that are 2-10 nm in size with a surface roughness ranging between 20 and 40 nm. UNCD films exhibit most of the outstanding properties of diamond such as an exceptional hardness, high mechanical strength, extremely low friction coefficient, chemical inertness, thermal stability and high electrical conductivity at room temperature when doped with nitrogen. While all these properties are intrinsic of the material, the required roughness scale of the UNCD film needs to be carefully tailored and customized on an application-by-application basis. For instance, a low roughness is key to manufacturing adequate micro-electromechanical systems (MEMS), wear-resistant low friction coatings, X-ray lithography masks and biomedical devices where stringent tolerance and topological precision are of utmost importance. Here, a new process for controlled smoothening of ultrananocrystalline diamond films using H2/O2 plasma is presented. Diamond films were exposed to oxygen, hydrogen and hydrogen/oxygen mixture (19:1) plasmas. Evolution of morphology and thickness were monitored ex-situ using scanning electron microscopy (SEM) while roughness was measured using atomic force microscopy (AFM). Neither pure oxygen nor pure hydrogen plasmas were found suitable for smoothening the films. The H2/O2 mixture plasma exposure resulted in a constant roughness decrement of 0.043 nm/min with a slow etching rate (0.07 µm/h) allowing for the control of the final film roughness without sacrificing too much of the material thickness.

Precision Recess of AlGaN/GaN with Controllable Etching Rate Using ICP-RIE Oxidation and Wet Etching

A method for highly controllable etching of AlGaN/GaN for the fabrication of high sensitivity HEMT based sensors is developed. The process consists of cyclic oxidation of nitride with O2 plasma using ICP-RIE etcher followed by wet etching of the oxidized layer. Previously reported cyclic oxidation-based GaN etching obtained very slow etching rate (∼0.38nm/cycle), limited by oxidation depth. The proposed approach allows fine control of the oxidation enabling the formation of accurately controlled recess of very thin (20∼30nm) barrier layers. With optimized power settings, etch rates from ∼0.6 to ∼11nm/cycle were obtained. AFM results did not show any increase in surface roughness after etching, indicating that surface quality of the etched layer was not affected by the etching process.

Foundation of the Reaction Mechanism of Deep Penetration and Low-Damage Acidizing Fluid

With the development of oil and gas resources to inefficient and multiple rounds measure reservoir development, with deep penetration characteristics of low-damage acid system acidification and acid fracturing technology more and more research is very important. Deep penetration and low-damage Acidizing system has some advantage which a slower etch rate, etch depth, no secondary damage, reservoir reconstruction obvious effect, acid-rock reaction kinetics parameters such testing and etching acid system mechanism is the basis of the depth of acidification and acid fracturing work. With deep penetration and low-damage Acidizing fluid system formulation, through new experimental device for etching experiments carried reservoir core, on the basis of Hekim distributed parameter model is proposed the reaction mechanism of deep penetration and low-damage acidizing fluid in the reservoir. The key consideration during the construction of deep penetration and low-damage acidizing fluid in the formation of continuous hydrolysis HF, detailed derivation of the body acid, HF concentration distribution and concentration distribution of mineral mathematical models and numerical models.

Modification of Electrode-Etchant for Sidewall Profile Control and Reduced Back-Channel Corrosion of Inverted-Staggered Metal-Oxide TFTs

For metal-oxide thin-film transistors (TFTs), the inverted staggered structure is hard to attain, given the sensitivity of metal-oxides to etchants used to define shapes of the source and drain electrodes. Here we present the development of a hydrogen peroxide (H2O2)-based (H2O, H2O2 and ammonium + azole) source/drain electrode wet-etchant, modified for reduced backchannel corrosion and metal electrode sidewall profile control. Diluting the etchant with ammonium decreases its acidity (from pH ∼3 to ∼5), resulting in decreased electrode etch rate and reduced metal-oxide backchannel corrosion as confirmed by corrosion potential measurements and X-ray photoelectron spectroscopy. However, slow etch rate yields a <20° non-uniform tapered electrode sidewall profile, which necessitates addition of an azole as an electrode sidewall corrosion inhibitor. The azole inhibits lateral corrosion, thereby achieving a uniform vertical etch, as it gets adsorbed at the metal sidewalls due to its chelating effect with metal ions that accumulate at the sidewalls during dip (immersion) etch. Metal oxide (amorphous-indium-gallium-zinc-oxide) TFTs fabricated by using the H2O2-based etchant exhibited turn-on voltages close to zero volts and subthreshold voltage slopes of ∼370 mV/dec.

Thermally driven metal-assisted chemical etching of GaAs with in-position and out-of-position catalyst

We have identified the characteristic stages of metal-assisted chemical etching of GaAs. Distinct changes in the surface topologies were classified into etching at incubation, out-of-position, and in-position of the metal catalyst. Thermally activated chemical etching and mass transport influence the catalytic reactions of electronic holes, oxidation of GaAs, and subsequent removal of porous GaAs. At low temperatures with a slow etch rate, only porous regions encircling the metal catalyst were etched on the GaAs. At mid temperatures, chemical etch occurred substantially out-of-position of the Au catalyst, forming arrays of craters on GaAs. At high temperatures, etching occurred in-position of the Au catalyst with an array of bumps on the GaAs. After a prolonged etch, high aspect ratio pillars were fabricated with a high vertical etch rate and the pillar diameters were shrunk to nano-scale with a controlled lateral etch. Three distinct stages that consecutively evolved during metal-assisted chemical etching of GaAs were determined to be caused by thermally driven electronic holes and chemical reactions.

MD simulations of low energy Clx+ ions interaction with ultrathin silicon layers for advanced etch processes

Molecular dynamics simulations of low-energy (5–100 eV) Cl+ and Cl2+ bombardment on (100) Si surfaces are performed to investigate the impact of plasma dissociation and very low-energy ions (5–10 eV) in chlorine pulsed plasmas used for silicon etch applications. Ion bombardment leads to an initial rapid chlorination of the Si surface followed by the formation of a stable SiClx mixed layer and a constant etch yield at steady state. The SiClx layer thickness increases with ion energy (from 0.7 ± 0.2 nm at 5 eV to 4 ± 0.5 nm at 100 eV) but decreases for Cl2+ bombardment (compared to Cl+), due to the fragmentation of Cl2+ molecular ions into atomic Cl species with reduced energies [one X eV Cl + &amp;lt;−&amp;gt; two 2X eV Cl2+]. The Si etch yield is larger for Cl2+ than Cl+ bombardment at high-energy (Ei &amp;gt; 25 eV) but larger for Cl+ than Cl2+ bombardment at low-energy (Ei &amp;lt; 25 eV) due to threshold effects. And the higher the ion energy, the less saturated the etch products. Results suggest that weakly dissociated chlorine plasmas (containing more Cl2+ than Cl+ ions) should lead to thinner SiClx mixed layers and lower Si etch yields if ion energies remains below 25 eV, which confirms the potential of pulsed plasmas to address etching challenges of ultrathin films transistors, in which slow etch rates and very controlled processes are required.

Sputtered thin film piezoelectric aluminum nitride as a functional MEMS material

A comprehensive study on the complete process for the fabrication of AlN-based MEMS sensors and actuators is presented. Varying the bias voltage during the reactive rf sputtering enables to adjust the stress level in AlN films in the range of about 2 GPa. For the first time the influence of the rf bias power on the whole set of piezoelectric parameters was investigated. It could be shown that the dielectric permittivity, dielectric loss, rocking curve width, effective longitudinal piezoelectric coefficient d 33,f and effective transverse piezoelectric coefficient e 31,f remained unchanged. Further it was observed that piezoelectric AlN films could be deposited at low process temperatures of only 200 °C. Moreover an increase in the e 31,f coefficient with thicker films could be stated. Finally cone formation during wet etching was observed and revealed a formation of {01–12} planes which exhibit a slow etching rate. The c-textured growth of AlN starts directly at the Pt interface.